During the distillation process of the water washing tower, due to low concentration throughout the distillation process, concentration at the top is 35% (VN), and discharge concentration at the bottom is 12-15% (VN). Under such condition, the alcohol distillation coefficient of almost all impurities is greater than 1; volatility coefficient of isoamyl alcohol is more than 5; volatility coefficient of acetaldehyde and acetic acid are 10 times higher. Content of impurities from water washing tower in gas phase is always greater than that in liquid phase. Impurities always move up, and gather at the top of the tower. Some impurities are discharged from aldehyde tube of the post-condenser, and some secondary alcohol enters the recovery tower. Liquor condensate of better quality from the front-condenser is reflux of the water washing tower. Weak liquor from the bottom of the washing tower to the rectifying tower is purified. The tower works at atmospheric pressure.